Great effort has been expended to provide alternatives of the finite sources of energy currently available. One alternative contemplated is the generation of electrical energy by conversion of solar radiation. The scientific literature has provided several examples of photoelectrochemical systems useful for the photoelectrolysis of water or the photo-oxidation of some suitable redox species. The theory underlying these systems and phenomena is reasonably well understood and is outlined, for example, in the following publications:
Gerischer, "Electrochemical Photo and Solar Cells Principles and Some Experiments", Electroanalytical Chemistry and Interfacial Electrochemistry, Vol. 58, pp. 263-274 (1975); Manassen et al., "Electrochemical, Solid State, Photochemical and Technological Aspects of Photoelectrochemical Energy Converters", Nature, Vol. 263, pp. 97-100 (1976); Ellis et al., "Study of N-Type Semiconducting Cadmium Chalcogenide-Based Photoelectrochemical Cells Employing Polychalcogenide Electrolytes", J. American Chemical Society, Vol. 99, pp. 2839-48 (1977); Wrighton et al., "Photo-Assisted Electrolysis of Water by Irradiation of a Titanium Dioxide Electrode", Proc. Nat. Acad. Sci., U.S.A., Vol. 72, No. 4, pp. 1518-1522 (1975); and Manassen et al. U.S. Pat. No. 4,064,326.
The photoelectrodes as generally described are semiconductors, n-type semiconductors being photo-anodes and p-type semiconductors being photo-cathodes. The semiconductors may be large bandgap materials, for example, n-TiO.sub.2 or small bandgap materials, for example n-GaAs. However, the application of photoelectrochemical semiconductor-electrolyte systems to the conversion of solar radiation to electrical energy suggests that semiconductors with bandgaps near 1.4 eV will be the most efficient with respect to the amount of solar radiation that can be usefully absorbed and converted to electrical energy. This consideration is well known from the established theory of solid state photovoltaic devices. Until recently, however, small bandgap materials could not be employed as photo-anodes for example, since irradiation in the presence of an electrolyte usually resulted in the photodissolution of the semiconductors. Several examples of redox couples are now known that will essentially eliminate the photo-dissolution of small bandgap semiconductors, such as disclosed in U.S. Pat. No. 4,172,925.
The usefulness for utilizing a ternary thin film compound semiconductor electrode layer in a solar radiation conversion device has also been recognized in the prior art. U.S. Pat. No. 3,218,203 discloses the structure and method for the production of epitaxial films of II-VI compounds by the reaction in the vapor phase of a group II component and a volatile group VI component in the presence of hydrogen, resulting in a solid state configuration. In this patent the ternary alloy disclosed is the same material as disclosed herein, namely an alloy including Cd, Se, and Te. In accordance with this prior patent, the alloy is formed by epitaxially depositing CdTe onto a CdTe crystal substrate and after some time the rate of depositing the CdTe is slowly reduced while at the same time CdSe is evaporated and deposited onto the substrate in a gradually increasing fashion. However, this method for forming the alloy is both time consuming and costly.